Positive OPK at 8DPO? What could that mean?

I know I ovulated about 8 days ago, positive OPK and ultrasound confirmed I dropped 2 eggs. Bloodwork also confirmed this. A few days ago OPK was completely negative (blank second line). The next day darker, then darker, and now 8DPO its positive again? Is it normal to have a surge twice? Dr is on vacation. (of course!) Nurse scheduled me for another ultrasound in about a week. Hope everything is okay! I was on Letrozole, so Im hoping its NOT possible to ovulate with meds then ovulate by myself!!!!